How do class D amplifiers work?

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A Class D amp works by taking the analog input signal and creating a PWM (pulse width modulation) replica of it-essentially a train of pulses, which correspond to the amplitude and frequency of the input signal. In its most basic form, a comparator circuit is used to match the input signal with the PWM signal. The PWM signal is then amplified by an output stage operating in switch mode, which is to say there are two states, on or off, at very high speed, corresponding to the PWM pulses. A linear amplifier's output stages, by comparison, see a continuous waveform and, to avoid distortion, are on for more than half the waveform (Class A/B) or for the complete waveform (Class A), thus greatly reducing efficiency and generating heat. The amplified PWM waveform is low pass filtered to recover the audio waveform and eliminate spurious ultrasonic noise before outputting it to the speakers. This process seems digital but is in fact analog in nature. The signal is not "digitized", i.e., ...